1. Field of the Invention
The present invention relates to a supervisory system for a radio transmission line, more particularly, it relates to a supervisory system which rapidly detects deterioration in the quality of the digital radio signal transmitted over the transmission line and performs transmission line switching when such deterioration is detected.
2. Description to the Prior Art
The recent trend in radio communication systems is for various information to be transformed into digital signals and the thus transformed information transmitted to a remote office through a digital radio transmission line. In such radio communication systems, the level of the received signal at the receiver side is often subjectd to extreme variations due to fading and the like occurring in the radio transmission line, and therefore, required transmission quality cannot be attained. In a conventional system, this quality deterioration is checked by continuous scanning an error rate in the normal and emergency lines. If the error rate in a normal line increases to a predetermined threshold value, transmission through the normal line is stopped and changed to an emergency line by, for example, space diversity, frequency diversity, or polarization wave diversity, so that the desired transmission quality can be assured.
A problem occurring in a prior art supervisory system, is that a line switching command cannot be issued rapidly. That is, too long a period must elapse before the line can be switched. Thus, by the time the line switching command is actually provided, the quality deterioration in the transmission line has become very serious; particularly when fading occurs not gradually but suddenly. The reason for the above will be descussed in detail hereinafter, but briefly, it is because the line switching command in a prior art system is generated in accordance with average error rate, which is determined every time a predetermined constant observation period is completed.